Process for Production of Cold-Rolled Steel Sheet Having Excellent Press Moldability, and Cold-Rolled Steel Sheet

ABSTRACT

Provided are a cold-rolled steel sheet which undergoes a small load at the time of cold-rolling, has excellent press formability and high strength, and a method of manufacturing the cold-rolled steel sheet. A hot-rolled steel sheet having the composition comprising by mass %, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance, and has a tensile strength of 1000 MPa or less is subjected to pickling and, is subjected to cold rolling at a total rolling reduction of 60% or more thus forming a cold-rolled steel sheet. A final continuous annealing treatment is performed at a soaking temperature of 750° C. or above and at a cooling rate of 3° C./s to 100° C./s so that the cold-rolled steel sheet which has a tensile strength of 1280 MPa or more, breaking elongation of 3% or more, and a thickness of 0.05 to 0.60 mm is manufactured.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a cold-rolledsteel sheet having excellent press formability and a cold-rolled steelsheet manufactured by the manufacturing method.

BACKGROUND ART

It is often the case where liquid crystal frame parts used in a mobilephone or a notebook-type personal computer, gasket parts for anautomobile engine and the like are formed by a cold press. Accordingly,a thin cold-rolled steel sheet which is used for forming these parts isrequired to have press formability.

Further, with respect to parts of an electronic device represented by anotebook-type personal computer, a mobile phone or a digital camera orparts such as a gasket for an automobile engine, these parts arerequired to satisfy a demand for the reduction of weight and theminiaturization. To realize the reduction of weight and for theminiaturization of these parts, it is necessary to make a steel sheetthin. On the other hand, when the steel sheet which is made thin has thesame strength as the steel sheet which is not made thin, the steel sheetmade thin cannot ensure a strength which a press product is required tohave. Accordingly, it is necessary to provide a high strength steelsheet having a thin gauge and a high strength.

In view of such a technical background, as a steel sheet which is usedin applications such as a reinforcing member for a bumper or a door ofan automobile, there has been proposed an ultra high-strength steelsheet having the martensite or bainite structure and also having atensile strength of 1180 MPa or more (see patent document 1).

Further, patent document 2 proposes a cold-rolled steel sheet for agasket material having excellent spring property which is manufacturedin such a manner where a steel sheet is heated at a recrystallizationtemperature or above in continuous annealing, is annealed after soaking,and is subjected to secondary cold rolling.

BACKGROUND ART DOCUMENT Patent Document

Patent document 1: Japanese Patent 3254106

Patent document 2: JP-A-9-194935

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, the high-strength steel sheets which are disclosed in patentdocument 1 and patent document 2 aim for the acquisition of highstrength so that ductility is lowered and cracks occur at the time ofpress forming thus giving rise to a drawback that press formability islow.

Further, to realize a thin gauge at the time of cold-rolling ahigh-strength hot-rolled steel sheet, a load at the time of cold rollingis increased thus giving rise to a drawback that productivity is impededdue to lowering of operability and the increase of a yield loss.

Accordingly, the present invention has been made to overcome theabove-mentioned drawbacks, and it is an object of the present inventionto provide a cold-rolled steel sheet which undergoes a small load at thetime of cold-rolling, has excellent press formability and has highstrength, and a method of manufacturing the cold-rolled steel sheet.

Means for Solving the Problems

(1) A method of manufacturing a cold-rolled steel sheet according to thepresent invention is characterized in that a hot-rolled steel sheetwhich has the composition comprising of, by mass %, 0.10 to 0.30 C, 0.2or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe andunavoidable impurities as a balance and has a tensile strength of 1000MPa or less is subjected to pickling and, thereafter, is subjected tocold rolling at a total rolling reduction of 60% or more thus forming acold-rolled steel sheet, and final continuous annealing treatment isperformed at a soaking temperature of 750° C. or above and at a coolingrate of 3° C./s to 100° C./s thus manufacturing a cold-rolled steelsheet having a tensile strength of 1280 MPa or more, breaking elongationof 3% or more and a thickness of 0.05 to 0.60 mm.

(2) The method of manufacturing a cold-rolled steel sheet according tothe present invention is, in the above-mentioned constitution (1),characterized in that a thickness of the hot-rolled steel sheet is 1. 2to 3.0 mm.

(3) A cold-rolled steel sheet according to the present invention ismanufactured by a method where a hot-rolled steel sheet which has thecomposition comprising of, by mass %, 0.10 to 0.30 C, 0.2 or more Mn,0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidableimpurities as a balance and has a tensile strength of 1000 MPa or lessis subjected to pickling and, thereafter, is subjected to cold rollingat a total rolling reduction of 60% or more thus forming a cold-rolledsteel sheet, and final continuous annealing treatment is performed at asoaking temperature of 750° C. or above and at a cooling rate of 3° C./sto 100° C./s, wherein the cold-rolled steel sheet has a tensile strengthof 1280 MPa or more, breaking elongation of 3% or more and a thicknessof 0.05 to 0.60 mm.

(4) The cold-rolled steel sheet according to the present invention is,in the above-mentioned constitution (3), a thickness of the hot-rolledsteel sheet is 1. 2 to 3.0 mm.

Advantageous Effects of the Invention

According to the present invention, it is possible to provide acold-rolled steel sheet which undergoes a small load at the time ofcold-rolling, and has excellent press formability while ensuring highstrength and ductility.

Further, according to the method of manufacturing a cold-rolled steelsheet of the present invention, it is also possible to provide acold-rolled steel sheet having both strength and formability which has athickness of 0.05 mm to 0.6 mm, a tensile strength of 1280 MPa or more,and breaking elongation of 3% or more, wherein these are conditionswhich a raw material used for manufacturing a gasket of a gasolineengine for an automobile, a liquid crystal frame or a frame part of anotebook-type personal computer, a mobile phone and a digital camera orthe like is required to have.

MODE FOR CARRYING OUT THE INVENTION <Composition of Hot-Rolled SteelSheet>

The composition of hot-rolled steel sheet is set to the following rangeby mass %.

C: 0.10 to 0.30, Mn: 0.2 or more, Ni: 0.01 or more, Mn+Ni: 0.5 to 2.5,Cr: 1.2 to 9.0, balance: Fe and unavoidable elements.

C is an important element for forming the low-temperature transformedstructure such as martensite in the steel sheet such that the steelsheet acquires high tensile strength.

When the content of C is extremely small, the required tensile strengthcannot be acquired and hence, a lower limit of the content of C is setto 0.10. On the other hand, when the content of C is extremely large, arolling load at the time of hot-rolling and a rolling load at the timeof cold-rolling are increased so that the productivity is impeded due tothe deterioration in shape or the like and hence, an upper limit of thecontent of C is set to 0.30. The content of C is more preferably set toa value which falls within a range from 0.15 to 0.25.

In the present invention, both Mn and Ni are elements which enhancehardenability and form the low-temperature transformed structure at thetime of continuous annealing, and Mn and Ni impart high tensile strengthto a cold-rolled steel sheet to which continuous annealing is applied.When a total amount of content of Mn and Ni is extremely small, thesteel sheet cannot acquire strength after continuous annealing isapplied to the steel sheet and hence, a lower limit of the total amountof content of Mn and Ni is set to 0.5. On the other hand, when the totalamount of content of Mn and Ni is extremely large, the above-mentionedadvantageous effect is saturated and, further, workability is lowereddue to the segregation and the like and hence, an upper limit of thetotal amount of content of Mn and Ni is set to 2.5.

Here, with respect to respective ranges of the content of Mn and thecontent of Ni, it is necessary for the steel sheet to contain 0.20 ormore of Mn since Mn prevents red heat caused by S which is an impurity,and it is necessary to add 0.01 or more of Ni to the steel sheet sinceNi ensures toughness of the steel sheet after heat treatment. Since Niis expensive, it is advantageous to adjust the total amount of contentof Mn and Ni by adjusting the content of Mn in view of cost.

Cr is also an element which can enhance hardenability, and Cr forms thelow-temperature transformed structure at the time of continuousannealing so that the steel sheet can acquire high tensile strength.When the content of Cr is extremely small, the steel sheet cannotacquire tensile strength after continuous annealing is applied to thesteel sheet and hence, a lower limit of the content of Cr is set to 1.2.On the other hand, when the content of Cr is extremely large, theabove-mentioned advantageous effect is saturated and, further, cost isincreased wastefully and hence, an upper limit of the content of Cr isset to 9.0. The content of Cr is more preferably set to a value whichfalls within a range from 2.0 to 5.5.

Si is an element effective for greatly increasing strength of acold-rolled steel sheet. The larger content of Si, the more easily theobject can be achieved. However, when the content of Si exceeds 2.0%, aload in cold-rolling is increased and a shape of the steel sheet isdeteriorated and hence, an upper limit value of the content of Si is setto 2.0%.

P is a component which makes crystal grains fine and hence, it isdesirable to add a fixed amount of P to the steel sheet for enhancingstrength of a cold-rolled steel sheet. However, P also causessegregation in a crystal grain boundary thus inducing brittleness of thesteel sheet. Accordingly, the content of P is set to 0.06% or less.

S is an impurity component which generates red-brittleness during hotrolling and hence, it is desirable that the steel sheet contains aslittle S as possible. However, it is impossible to completely preventthe mixing of S from a raw material or the like into a steel sheet, andthe desulfurization in the manufacturing steps is also limited andhence, it is unavoidable that S remains in the steel sheet to someextent. The red-brittleness caused by a small amount of residual S canbe alleviated by Mn and hence, an upper limit value of the content of Sis set to 0.06%.

A fixed amount of Cu may be added to the steel sheet for enhancing thestrength of a steel sheet by solid solution strengthening or byprecipitation strengthening. On the other hand, however, there exists apossibility that Cu will bring about brittleness in the steel sheet atthe time of hot rolling. Accordingly, an upper limit of Cu is set to0.5%.

Al is added to a steel bath as a deoxidizing agent in making steel. Alreacts with solid solution N and segregates as AlN thus contributing tomaking the crystal grain fine. On the other hand, when the content of Alexceeds 0.10%, the solidification of N becomes conspicuous thus loweringsolid solution strengthening brought about by N and hence, the contentof Al is set to 0.10% or less.

The addition of Ti is effective for making the crystal grain fine, forsuppressing a grain growth, for enhancing corrosion resistance and thelike. However, even when the content of Ti added to the steel sheet isexcessively large, the advantageous effect is saturated and hence, thecontent of Ti is set to 0.30% or less.

N is, in the same manner as C and Mn, a component necessary forimparting high strength to a cold-rolled steel sheet and for increasingdurability of the cold-rolled steel sheet. However, setting the contentof N to less than 0.002% is difficult in terms of making steel. On theother hand, the addition of the content of N which exceeds 0.015%remarkably lowers a yield rate of ferro-nitride at the time of makingsteel thus making the steel sheet unstable, and also remarkablydeteriorates anisotropy of the steel sheet at the time of press forming.

Further, cracks occur on a surface of a continuously-cast slab and thecracks become a defect in casting. Accordingly, the content of N isdesirably set to 0.015% or less in the present invention.

<Manufacturing Steps>

Hereinafter, a method of manufacturing a cold-rolled steel sheetaccording to the present invention is explained.

<Making Steel>

Raw materials are melted in a converter or an electric furnace toproduce molten steel and molten steel is cast for producing a slab whosecomposition is adjusted to the above-mentioned range.

<Hot Rolling>

A slab whose components are adjusted to the above-mentioned range isformed into a steel sheet having a sheet thickness of 1.2 to 3.0 mm byhot-rolling. When the sheet thickness is set small, a load at the timeof hot-rolling is increased. Accordingly, a lower limit of the sheetthickness is set to 1.2. On the other hand, when the sheet thickness isset large, a load at the time of cold-rolling which is performed afterhot-rolling is increased. Accordingly, an upper limit of the sheetthickness is set to 3.0.

In the hot-rolling step, a heat temperature of the slab having theabove-mentioned composition is set to 1100° C. or above, and a coilingtemperature (CT) is set to 600° C. or above. When the heatingtemperature of the slab is below 1100° C., the positive decompositionsolid solution of N becomes insufficient and a load at the time ofhot-rolling is increased. Accordingly, such a heating temperature is notpreferable.

Further, the winding temperature is set to a value which falls within arange from 600° C. to 800° C. When the winding temperature is low,strength of the hot-rolled steel sheet is largely increased so that sucha steel sheet is not preferable for cold-rolling. Accordingly, a lowerlimit of the winding temperature is set to 600° C.

On the other hand, when the winding temperature exceeds 800° C., thegeneration of scales is accelerated at the time of hot rolling so that aload at the time of descaling by pickling is increased whereby an upperlimit of the winding temperature is set to 800° C.

The hot-rolled steel sheet which is manufactured under theabove-mentioned condition may have a tensile strength of 1000 MPa orless. This is because when tensile strength of the hot-rolled steelsheet exceeds 1000 MPa, a rolling load at the time of cold rolling isincreased so that the tensile strength exceeding 1000 MPa is notpreferable.

<Pickling>

Scales generated on a surface of the steel sheet at the time ofhot-rolling are removed in an acid bath in accordance with a normalmethod and, thereafter, the steel sheet is subjected to cold-rolling andcontinuous annealing.

<Cold-Rolling>

Cold-rolling is applied to the steel sheet once or several times at arolling reduction of 60% or more in total until the steel sheet, as aproduct, acquires a predetermined product thickness of 0.05 to 0.6 mm.This is because when the product thickness becomes 0.05 mm or less,rigidity of the steel sheet is lowered so that there may be a case wherewhen the steel sheet is used for manufacturing products such as a gasketof a gasoline engine of an automobile, a liquid crystal frame or a framepart of a notebook-type personal computer, a mobile phone and a digitalcamera, the shape of the product is liable to be deformed and hence, theproduct cannot be manufactured.

On the other hand, when the product thickness becomes 0.6 mm or more,there may be a case where when the steel sheet is used for manufacturinga product such as a gasket of a gasoline engine of an automobile, aliquid crystal frame or a frame part of a notebook-type personalcomputer, a mobile phone and a digital camera, a weight of the productbecomes larger than a designed value or the miniaturization of theproduct cannot be realized.

Although an upper limit of the total rolling reduction in cold-rollingis not particularly limited, the upper limit of the total rollingreduction is set to 98%.

Further, by applying continuous annealing at a temperature of 500° C. orabove or batch annealing at a temperature of 500° C. or above to thesteel sheet after cold-rolling, the rolled sheet which is hardened bywork hardening is softened so that cold-rolling can be applied to thesteel sheet again.

A method of cold-rolling and the number of times that cold-rolling isperformed are not particularly designated, and the method and the numberof times that cold-rolling is performed can be suitably selected inaccordance with a target sheet thickness.

<Continuous Annealing>

Continuous annealing is applied to the steel sheet for removing strainsin the sheet generated at the time of cold-rolling. When cold-rolling isperformed plural times, continuous annealing can be performed in eachcold-rolling.

According to the present invention, in the final continuous annealing,the steel sheet is soaked and held at a soaking temperature of 750° C.or above for 1 second or more and 100 seconds or less. When the soakingtemperature is below 750° C., a temperature of the steel sheet does notexceed an Ae3 transformation temperature and hence, the steel sheetcannot acquire sufficient tensile strength. On the other hand, even whenthe soaking temperature exceeds 1000° C., there is no particularadvantageous effect, and such a temperature is wasteful from theindustrial viewpoint and hence, an upper limit of the soakingtemperature is set to 1000° C.

In the final continuous annealing, apart or the entire steel sheet isformed into austenite by heating the steel sheet and, thereafter, theseparts are transformed into martensite or the like by cooling after suchheating.

According to the present invention, corresponding to an amount ofmartensite and an amount of alloy element, the steel sheet can acquire apredetermined strength.

After the steel sheet is soaked and held at a soaking temperature of750° C. or above for 1 second or more and 100 seconds or less, the steelsheet is cooled at a cooling rate of 3° C./s to 100° C./s. Due to suchcooling, the austenite structure of the steel sheet is formed intostructures such as martensite, tempered martensite and bainite. When thecooling rate is 3° C./s or less, the formation of the low-temperaturetransformed structure such as martensite becomes insufficient and hence,the steel sheet cannot acquire sufficient strength. On the other hand,when the cooling rate exceeds 100° C./s, although the steel sheet canacquire required strength, a shape of the steel sheet is deterioratedand hence, such a steel sheet is not preferable for press application.

In the present invention, after the continuous annealing, a steel sheetis cooled at a cooling rate which is remarkably slow compared with acooling rate for general quenching using water or the like which isperformed for ensuring a shape so that the low-temperature transformedstructure is formed and thereby a high strength steel sheet is acquired.

With respect to Mn and Ni which are effective austenite stabilizingelements used as general quenching elements, it is necessary to add aconsiderable amount of Mn and Ni to the steel sheet to acquire arequired strength at the above-mentioned cooling rate. However, theaddition of a considerable amount of Mn and Ni to the steel sheetlargely increases strength of the steel sheet after hot-rolling so thata load at the time of cold-rolling is increased whereby productivity isimpeded thus inducing an increase in product cost.

According to the present invention, the content of Mn and the content ofNi are set relatively small, and Cr which is a ferrite stabilizingelement is added to the steel sheet so as to form a composite and hence,it is possible to make the steel sheet relatively soft after hot-rollingand to manufacture a high-strength steel sheet by continuous annealingwhich is performed after cold-rolling whereby a load in the manufactureof the steel sheet can be significantly reduced.

Further, when necessary, tempering treatment below 600° C. may beapplied to the steel sheet after the steel sheet is cooled after finalcontinuous annealing. In the present invention, however, the steel sheetis formed into the quenched structure at a relatively low cooling rateand hence, an effect substantially equal to an effect obtained bytempering treatment can be acquired at the time of cooling whereby thetempering step can be omitted. The omission of the tempering steplargely contributes to the reduction of a load in the manufacture of thesteel sheet.

Due to the above-mentioned continuous annealing applied to the steelsheet, tensile strength of a cold-rolled steel sheet acquired by themanufacturing method of the present invention can be increased to 1280MPa or more. Since the cold-rolled steel sheet has such strength, whenthe cold-rolled steel sheet is used for manufacturing a gasket of agasoline engine of an automobile, there is no possibility that a gasleak will occur.

Further, when a sheet thickness of the cold-rolled steel sheet is madesmall for reduction of weight and the cold-rolled steel sheet is usedfor manufacturing a liquid crystal frame or frame parts of anotebook-type personal computer, a mobile phone, or a digital camera,and particularly when the cold-rolled steel sheet is used in mobileapplications, these parts can ensure the required rigidity as the parts.

Further, in the manufacturing method of the present invention, breakingelongation of a cold-rolled steel sheet can be set to 3% or more. Avalue of breaking elongation indicative of ductility is important when acold-rolled steel sheet is press-formed for manufacturing products suchas gaskets for an automobile engine and electronic parts. When breakingelongation is less than 3%, cracks are liable to occur in portions whichare subjected to some degree of bulging forming, bending at 90° or thelike.

A steel sheet manufactured by the above-mentioned manufacturing methodhas the structure formed of martensite, tempered martensite, bainitestructure and the like by a volume % of 60% or more, has high tensilestrength of 1280 MPa or more, and ensures ductility.

<Temper Rolling or the Like>

The steel sheet obtained in this manner is, when necessary, subjected totemper rolling for surface roughness adjustment or subjected to electroplating and chemical conversion coating using Zn, Ni or the like forrust prevention.

<Press Forming>

The steel sheet obtained by the manufacturing method of the presentinvention can be used as a raw material for press forming. However, thesteel sheet obtained by the manufacturing method of the presentinvention largely differs from a generally-used cold-rolled steel sheetin a strength range and hence, it is necessary to take into accountspring-back or the like.

EXAMPLES

To explain a cold-rolled steel sheet of the present invention in furtherdetail in conjunction with examples and comparison examples, slabshaving compositions corresponding to specimens 1 to 18 shown in Table 1are prepared. The specimens 1 to 11, 17, 18 are slabs having thecompositions which fall within a composition range of the presentinvention, while the specimens 12 to 16 are slabs which do not fallwithin the composition range of the present invention.

In hot-rolling step, the slabs which are adjusted to the compositionscorresponding to specimens 1 to 18 are heated at a temperature of 1230°C., and hot-rolled steel sheets having a sheet thickness of 2.0 mm aremanufactured at winding temperatures shown in Table 2. Properties ofthese hot-rolled steel sheets are shown in Table 2.

Next, after pickling these hot-rolled steel sheets, the hot-rolled steelsheets are cold-rolled to a thickness of 0.5 mm. Thereafter, in acontinuous annealing step, the cold-rolled steel sheets are soaked andheld at a soaking temperature of 900° C. for 20 seconds and, thereafter,the cold-rolled steel sheets are cooled at a cooling rate of 20° C./sthus forming cold-rolled steel sheets. Properties of these cold-rolledsteel sheets are shown in Table 2.

As can be understood from Table 2, in the examples 1 to 15, tensilestrength of the hot-rolled steel sheets is 1000 MPa or less so that athickness of the steel sheet can be reduced to targeted 0.5 mm in coldrolling which is performed after hot-rolling, and tensile strength ofthe cold-rolled steel sheet can be set to 1280 MPa or more and breakingelongation can be set to 3% or more.

In the examples 3, 4, by changing a cooling rate after soaking to 3°C./s, 100° C./s respectively, cold-rolled steel sheets which havetensile strengths of 1320 MPa and 1405 MPa and have breaking elongationof 6.2% and 6.0% respectively can be obtained.

A hot-rolled steel sheet of a comparison example 1 is manufactured usingthe specimen 1 from which a hot-rolled steel sheet of the example 1 isalso manufactured. However, the winding temperature is set to a lowtemperature of 480° C. and hence, tensile strength of a hot-rolled steelsheet is increased to 1108 MPa whereby the hot-rolled steel sheetbecomes hard. Accordingly, a thickness of the steel sheet cannot bereduced to targeted 0.5 mm in cold rolling which is performed afterhot-rolling, and when a rolling load and the number of times thatrolling is performed are increased, cracks occur in the steel sheet andhence, cold-rolling is stopped.

A hot-rolled steel sheet of a comparison example 2 is manufactured usingthe specimen 2 from which hot-rolled steel sheets of the examples 2 to 4are also manufactured. However, the winding temperature is set to a lowtemperature of 500° C. and hence, tensile strength of a hot-rolled steelsheet is increased to 1216 MPa whereby the hot-rolled steel sheetbecomes hard. Accordingly, a thickness of the steel sheet cannot bereduced to targeted 0.5 mm in cold rolling which is performed afterhot-rolling, and when a rolling load and the number of times thatrolling is performed are increased, cracks occur in the steel sheet andhence, cold-rolling is stopped.

The content of C is set to 0.10 or less in comparison examples 3, 4, thecontent of Cr is set to 1.2% or less in a comparison example 5, and thecontent of Mn and the content of Ni are small and the content of Mn+Niis set to 0.5% or less in a comparison example 7. Accordingly, in therespective comparison examples, tensile strength of a cold-rolled steelsheet becomes 1280 MPa or less so that when the cold-rolled steel sheetis used for manufacturing gaskets or frame parts, the cold-rolled steelsheet has insufficient strength whereby the cold-rolled steel sheet isnot applicable to the manufacture of the gaskets and the frame parts.

In a comparison example 6, the total content of Mn+Ni is large, that is,3.4 and hence, tensile strength of the hot-rolled steel sheet isincreased whereby the steel sheet cannot be rolled to 0.5 mm which is atarget thickness in cold-rolling which is performed after hot-rolling.To be more specific, due to high hardness, a thickness of the hot-rolledsteel sheet cannot be reduced even when the hot-rolled steel sheet isrolled, or when a rolling load or the number of times that rolling isperformed is increased, cracks occur in the steel sheet so that thecold-rolled steel sheet cannot be manufactured.

TABLE 1 composition value [wt %] slab specimen c Si Mn P S Cu Ni Cr S.AlTi N Mn + Ni 1 0.15 0.18 0.8 0.004 0.005 0.19 0.1 3.1 0.019 0.00 0.0090.9 example 1 comparison example 1 2 0.14 0.17 1.6 0.006 0.003 0.20 0.12.5 0.026 0.10 0.010 1.7 example 2 example 3 example 4 comparisonexample 2 3 0.15 0.18 1.61 0.006 0.005 0.19 0.10 3.1 0.024 0.00 0.0121.7 example 5 4 0.15 0.20 0.89 0.004 0.003 0.19 1.02 2.6 0.026 0.100.010 1.9 example 6 5 0.18 1.12 0.78 0.003 0.003 0.05 0.07 2.5 0.0100.03 0.010 0.9 example 7 6 0.15 0.21 0.82 0.005 0.006 0.19 0.11 5.10.017 0.00 0.013 0.9 example 8 7 0.14 0.19 0.42 0.015 0.005 0.31 0.113.0 0.031 0.00 0.008 0.5 example 9 8 0.16 0.28 1 60 0.012 0.005 0.190.66 3.0 0.012 0.00 0.011 2.3 example 10 9 0.10 0.31 1.62 0.005 0.0020.18 0.13 2.9 0.025 0.00 0.009 1.8 example 11 10 0.30 0.55 1.19 0.0030.004 0.15 0.12 2.0 0.031 0.00 0.008 1.3 example 12 11 0.11 0.25 0.440.011 0.010 0.11 0.05 8.9 0.025 0.00 0.011 0.5 example 13 12 0.08 0.210.82 0.006 0.005 0.19 0.11 7.2 0.008 0.00 0.016 0.9 comparison example 313 0.07 0.18 1.60 0.006 0.003 0.19 0.10 2.5 0.025 0.10 0.010 1.7comparison example 4 14 0.18 1.17 0.72 0.004 0.005 0.12 0.09 1.0 0.0170.11 0.008 0.8 comparison example 5 15 0.15 0.20 0.84 0.004 0.003 0.182.51 2.6 0.023 0.10 0.009 3.4 comparison example 6 16 0.19 1.17 0.360.001 0.005 0.05 0.06 2.1 0.020 0.01 0.010 0.4 comparison example 7 170.18 0.21 1.49 0.005 0.003 0.19 0.09 1.5 0.022 0.00 0.006 1.6 example 1418 0.18 0.22 1.82 0.004 0.004 0.18 0.11 1.2 0.028 0.01 0.009 1.9 example15

TABLE 2 mechanical properties of hot-rolled steel sheet properties aftercontinuous annealing winding tensile breaking cooling tensile breakingtemperature strength elongation temperature rate strength elongation [°C.] [MPa] [%] [° C.] [° C./s] [MPa] [%] example 1 650 684 20.8 900 201304 6.1 comparison 480 1108 12.2 — 20 — — example 1 example 2 650 64619.6 900 20 1348 6.1 example 3 900 3 1320 6.2 example 4 900 100 1405 6.0comparison 500 1216 10.4 — 20 — — example 2 example 5 650 585 22.8 90020 1392 6.3 Example 6 650 672 19.4 900 20 1439 7.1 example 7 620 63518.8 900 20 1501 6.9 example 8 650 621 21.7 900 20 1359 6.0 example 9650 584 22.2 900 20 1371 6.4 example 10 650 712 17.7 900 20 1381 6.2example 11 650 532 22.8 900 20 1327 6.3 example 12 650 694 19.1 900 201588 6.0 example 13 650 579 18.9 900 20 1388 6.7 comparison 650 547 26.7900 20 1214 6.4 example 3 comparison 650 625 19.8 900 20 1052 7.4example 4 comparison 620 532 21.3 900 20  853 18.5  example 5 comparison650 1282 7.7 900 20 — — example 6 comparison 650 551 20.9 900 20 12278.3 example 7 example 14 650 588 21.1 900 20 1388 6.1 example 15 650 65519.2 900 20 1421 6.5

According to the present invention, it is possible to provide acold-rolled steel sheet which undergoes a small load at the time ofcold-rolling, has excellent press formability while ensuring highstrength and ductility. Further, it is also possible to provide acold-rolled steel sheet having both strength and formability which has athickness of 0.05 mm to 0.6 mm, a tensile strength of 1280 MPa or more,and breaking elongation of 3% or more, wherein these are conditionswhich a raw material used for manufacturing a gasket of a gasolineengine for an automobile, a liquid crystal frame or a frame part of anotebook-type personal computer, a mobile phone and a digital camera orthe like is required to have. Accordingly, the present invention has anextremely high industrial applicability.

1. A method of manufacturing a cold-rolled steel sheet in which ahot-rolled steel sheet which has a composition by mass % comprising:0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2to 9.0 Cr, and Fe and unavoidable impurities as a balance, and has atensile strength of 1000 MPa or less, is subjected to pickling and,thereafter, is subjected to cold rolling at a total rolling reduction of60% or more, thus forming a cold-rolled steel sheet, and finalcontinuous annealing treatment is performed at a soaking temperature of750° C. or above and at a cooling rate of 3° C./s to 100° C./s, thusmanufacturing a cold-rolled steel sheet having a tensile strength of1280 MPa or more, breaking elongation of 3% or more and a thickness of0.05 to 0.60 mm.
 2. The method of manufacturing a cold-rolled steelsheet according to claim 1, wherein a thickness of the hot-rolled steelsheet is 1.2 to 3.0 mm.
 3. A cold-rolled steel sheet which ismanufactured by a method where a hot-rolled steel sheet which has acomposition by mass % comprising: 0.10 to 0.30 C, 0.2 or more Mn, 0.01or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidableimpurities as a balance, and has a tensile strength of 1000 MPa or less,is subjected to pickling and, thereafter, is subjected to cold rollingat a total rolling reduction of 60% or more thus forming a cold-rolledsteel sheet, and final continuous annealing treatment is performed at asoaking temperature of 750° C. or above and at a cooling rate of 3° C./sto 100° C./s, wherein the cold-rolled steel sheet has a tensile strengthof 1280 MPa or more, breaking elongation of 3% or more and a thicknessof 0.05 to 0.60 mm.
 4. The cold-rolled steel sheet according to claim 3,wherein a thickness of the hot-rolled steel sheet is 1.2 to 3.0 mm.